Compositions used in the manufacture of crosslinkable articles, such as heat resistant wire and cable coatings and molded parts and accessories, typically require cross-linking after final shaping. Various crosslinking methods are practiced in the art, two of which are in wide usage, i.e., peroxide crosslinking and moisture cure (the latter of which usually employs a silane grafted or copolymerized polyolefin).
Moisture cure systems have the advantage in that they can be processed within a wide range of melt temperatures but are generally limited to thin wall constructions because the crosslinking relies on diffusion of external moisture into the article. Peroxide cure compositions are preferred for thick wall constructions, e.g. medium voltage (MV) cable insulation and molded cable accessories. These curable compounds need to be processed at temperatures which are below the peroxide decomposition temperature in order to avoid premature crosslinking (scorch) prior to forming the article. Once the article is formed, it needs to be heated uniformly to the peroxide decomposition temperature, and then held at that temperature for the time necessary to achieve the desired level of crosslinking. This can keep the production rate for such articles low due to poor heat transfer through the article walls. Furthermore, once the article is cooled, peroxide decomposition slows down to negligible levels; thus any significant crosslinking comes to an end. The combined problems of scorch and long heating and cure times (whether in-mold cure time or residence time in a continuous vulcanization tube) lead to long manufacturing cycles, and thus low productivity (units per time).
Flexibility is desired in crosslinked polyolefin articles, particularly in wire and cable applications. Flexibility in wire and cable coating promotes proper cable reeling as well as handling during cable joining. Flexibility in wire & cable accessories promotes ease of installation over a broad range of cable sizes.
Known are oil extender additives for improving flexibility to crosslinked articles. Addition of oil extender to crosslinked articles, however, has its drawbacks. In particular, addition of oil extender results in a decrease in dielectric strength (ACBD) for the crosslinked article.
A need exists for flexible crosslinked polyolefin articles with high dielectric strength. A need further exists for flexible crosslinked polyolefin articles for wire and cable applications.